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Polymer International 43 (1997) 210È216
Preparation, Characterization and
Biodegradation Characteristics of
Poly(adipic anhydride-co -D,L -lactide)
K. J. Zhu* & Yuan Lei
Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, PeopleÏs Republic of China
(Received 13 November 1996 ; accepted 19 December 1996)
Abstract : Poly(adipic anhydride-co-D,L-lactide) (P(AA-LA)) has been synthesized
by ring-opening polymerization of adipic anhydride (AA) and D,L-lactide (LA)
using stannous octoate as catalyst in bulk and in solution. The copolymers were
characterized by IR, nuclear magnetic resonance, gel permeation chromatography and di†erential scanning calorimetry. The physical properties can be
tailored by varying the copolymer compositions, and showed low glass transition
temperature, melting temperature and good solubility in common solvents. In
vitro tests showed that after rapid weight loss in the Ðrst day, a constant degradation rate was observed. The release proÐles of model drugs, bovine serum
albumin and norethindrone over 16 days followed closely that of the degradation
of copolymers containing higher amounts of AA (AA [ 64 mol%), suggesting
that the release mechanism was controlled predominantly by surface erosion.
However, a large deviation from the close correlation of polymer degradation
and drug release was observed for copolymers containing lower amounts of AA
(\30 mol%). These materials may be useful in protein delivery systems.
Polym. Int. 43, 210È216 (1997)
No. of Figures : 10. No. of Tables 3.
No. of Refs : 14
Key words : Poly(adipic anhydride-co-D,L-lactide), surface erosion copolymers,
drug controlled release
and protein drugs. We assumed that these kinds of
matrices might facilitate the stability of peptide and
protein in the release process, which is a major problem
in the area of peptide and protein controlled release.8
For this purpose, we prepared a group of novel copolymers which exhibited surface biodegradable characteristics. In this communication we report some
preliminary results on the preparation and degradation
characteristics of poly(adipic anhydride-co-D,L-lactide)
(P(AA-LA)).
INTRODUCTION
Controlled release matrices composed of biodegradable
polymers such as poly(D,L-lactide), poly(glycolide) and
their copolymers generally degrade in a homogeneous
manner.1,2 This leads to a progressive loosening of the
matrix from the surface and inner layers, which causes
change in both permeability and mechanical strength of
the devices. It would be desirable for a matrix to be
degraded heterogeneously from the surface to inner
layers. Such erosion would lead to zero-order drug
release.3 Poly(orthoester)4 and poly(anhydride) have
been reported as bioerodible materials ; the latter especially has been extensively studied by Langer et al.5h7
Recently we studied the preparation of surface biodegradable polymers with hydrophobic characteristics in a
search for new matrices for controlled release of peptide
EXPERIMENTAL
Synthesis of monomers
D,L-Lactide (LA) was synthesized according to the literature.9 Crude products were recrystallized three times
from ethyl acetate, giving material with a melting point
* To whom all correspondence should be addressed.
210
( 1997 SCI. Polymer International 0959-8103/97/$17.50
Printed in Great Britain
Characterization and biodegradation of P(AA-L A)
211
5DXC. Samples were either Ðlm-cast in chloroform on
KBr plates or pressed into KBr pellets. The NMR
spectra were recorded on a JEOL 90Q NMR spectrometer at room temperature in CDCl with tetra3
methylsilane as the internal standard. The molecular
weight was determined by GPC (Waters-208 with polystyrene standards, k-Styragel columns of 103, 104 and
105 Ó, chloroform as solvent, Ñow rate 1É5 ml min~1). A
Perkin Elmer DSC-7 was used for the determination of
transition temperature. The sample size was about
10 mg and the heating rate was 10¡C min~1. Scanning
electron micrographs were obtained on a JSM-T20
instrument. The specimens were sputter-coated with
goldÈpalladium for visualization in the scanning electron microscope.
of 125È126¡C. Adipic anhydride (AA) was synthesized
as described by Hill.10 Adipic acid (50 g) and acetic
anhydride (40 ml) were reÑuxed together for 4 h. The
acetic acid formed in the reaction and the excess acetic
anhydride were removed by distillation in a vacuum.
The distillate was collected between 105 and 120¡C
under vacuum (0É1 mmHg). The solid phase of the distillate was separated by centrifuging, and the liquid was
redistilled. Colourless AA was obtained (boiling point
98È100¡C at 0É1 mmHg, yield c. 40%), and sealed in a
dried glass tube for use.
Polymerization
For polymerization in bulk, well dried LA and AA were
placed into a polymerization tube under nitrogen atmosphere, and stannous octoate (0É2 mol%, dissolved in
petroleum ether) was added. The tube was connected to
a vacuum system and heated at 60¡C for 30 min to
remove solvent, the temperature was then raised to
130¡C and the tube was sealed. The polymerization was
carried out at 130¡C for 48 h.
For polymerization in solution, well dried monomers,
toluene and catalyst were placed into a polymerization
tube and sealed under reduced pressure, then heated at
90¡C for 20 h. The product mixtures were precipitated
from n-hexane.
The crude products obtained from both bulk and
solution polymerization were extracted with a solvent
mixture of ethyl acetate and n-hexane (2 : 1 v/v) to
remove possible LA and PLA. Then the products were
further extracted with a solvent mixture of methylene
chloride and ethyl acetate (1 : 5É5 v/v) to remove possible AA and PAA. The characteristics of the copolymers are given in Table 1.
Degradation and drug release in vitro
Drug-incorporated matrices were formulated as follows.
The micronized copolymers were sieved into a particle
size range of 90È150 km and mixed with the same size
range of model drugs. The mixture was pressed into circular discs (diameter 15 mm, thickness 2 mm) under a
pressure of 500 kg cm~2 at 60¡C. To study the degradation behaviour of the copolymers, the same procedures
were conducted as for sample preparation, except for
drug mixing.
Degradation of the copolymers in vitro was measured
by immersion of samples in phosphate-bu†ered saline
(PBS ; pH 7É4) at 37¡C ; samples were recovered periodically, and weight loss and molecular weight were determined. The drug release kinetics were followed by
measuring the UV absorbance at 240 nm for norethindrone (NET) and at 280 nm for bovine serum
albumin (BSA).11 The degradation and release experiments were done in triplicate.
Characterization
RESULTS AND DISCUSSION
The copolymers were characterized by IR, nuclear magnetic resonance (NMR), gel permeation chromatography (GPC) and di†erential scanning calorimetry.
The Fourier transform infrared (FTIR) spectra were
obtained from a Nicolet FTIR spectrometer model
Polymerization
The polymerization data are partially shown in Tables
2 and 3. It was found that the copolymerization of AA
TABLE 1. Characteristics of PAA and P(AA-LA) copolymersa
Sample code
Composition
(AA/LA by mole)
M1
n
(Ã104)
M1
w
(Ã104)
M1 /M1
w n
T
g
(¡C)
T
m
(¡C)
PAA
P(AA-LA) É 82
P(AA-LA)-64
P(AA-LA)-45
P(AA-LA)-30
P(AA-LA)-15
PLA
100/0
82/18
64/36
45/55
30/70
15/85
0/100
1·18
1·31
1·36
1·42
1·55
1·61
1·66
2·59
2·75
2·99
3·26
3·25
3·86
3·65
2·2
2·1
2·2
2·3
2·1
2·4
2·2
–
–
–
36
42
49
56
71
58
57
55
–
–
–
a The compositions were determined by 1H NMR ; molecular weights were measured by
GPC using chloroform as solvent at 25¡C ; T and T values were obtained by DSC.
g
m
POLYMER INTERNATIONAL VOL. 43, NO. 3, 1997
K. J. Zhu, Y . L ei
212
TABLE 2. Partial data of bulk copolymerization of adipic anhydride (AA) and lactide
(LA)a
AA/LA
(by mole)
Catalyst
conc. (Ã10É3, mol%)
Polym.
temp. (¡C)
Polym.
time (h)
Yield
(%)
M1
w
(Ã104)
M1 /M1
w n
0·2
0·3
0·5
1·0
1·5
0·5
0·5
0·5
0·5
0·5
2·0
3·5
3·5
3·5
3·5
3·5
3·5
3·5
12·0
3·5
120
120
120
120
120
110
130
140
120
120
30
30
30
30
30
48
30
30
30
48
82
80
76
73
69
85
75
65
78
82
3·2
3·3
3·0
2·7
2·2
3·6
2·9
2·5
2·8
2·6
2·2
2·3
2·0
2·1
2·3
2·4
2·0
2·2
2·3
2·2
a Sn(oct) was used as catalyst ; M1 was measured by GPC using chloroform as solvent at 25¡C.
2
w
and LA can be carried out in both bulk and solution,
but the molecular weight of the resulting polymers was
higher in bulk than in solution. Several catalysts were
e†ective for AA and LA reaction ; however, AlEt ÈH O
3 2
and Al(o-iPr) were more efficient (Table 3). If toxicity
3
of catalyst was of concern, Sn(oct) was preferred for
2
use. The concentration of catalysts in the range 2 to
12 ] 10~3 mol% had no signiÐcant e†ect on the yield
and molecular weight of the copolymers. A reaction
temperature in the range 110 to 130¡C for bulk and 70
to 90¡C for solution polymerization was preferred.
When the temperature was over 140¡C in bulk polymerization, the product became dark and the molecular
weight decreased. A non-polar solvent (such as toluene)
was more efficient than a polar solvent (such as
dimethylsulphoxide) in solution polymerization. It was
also noticed that the AA fraction in the copolymer was
higher than that in the feed, indicating that AA was
more reactive than LA. The reactivity ratios were calcu-
lated by the FinemanÈRoss method12 as r (AA) \ 3É46
1
and r (LA) \ 0É46.
2
Characterization
Figure 1 shows the IR spectrum of P(AA-LA). The
characteristic absorption bands due to the anhydride
group are observed at 1810 cm~1 and 1745 cm~1, the
latter being mixed with the symmetric stretching of the
ester group of LA. The other absorption bands
observed in the 3020È2950 cm~1 region are characteristic of CwH stretching. The bands at 1240 cm~1 were
attributed to wCOOw stretching, and those at 1030È
1085 cm~1 to CwO stretching of LA and AA.
The 1H and 13C NMR spectra of P(AA-LA) are
shown in Figs 2 and 3, respectively. By comparison with
the NMR spectra of PAA and PLA, the peaks may be
assigned as follows : 5É18 for [ CHw of LA, 2É20 for
wCOCH w of AA and 1É40È1É52 ppm for overlapping
2
TABLE 3. Partial data of solution copolymerization of adipic anhydride
(AA) and lactide (LA)a
Catalyst
Sn(oct)
2
AlEt –H O
3 2
Al(o -i Pr)
3
ZnCl
2
Solvent
Polym.
temp. (¡C)
Polym
time (h)
Yield
(%)
M1
w
(Ã104)
M1 /M1
w n
Toluene
Toluene
THF
Dioxane
DMF
DMSO
Toluene
Toluene
Toluene
70
90
70
90
90
90
90
90
90
35
20
20
20
20
20
20
20
20
75
78
63
57
43
21
82
77
trace
2·1
2·2
1·8
1·5
1·1
0·8
2·3
2·1
–
2·2
2·1
2·2
2·3
2·3
2·2
2·4
2·3
–
a AA/LA was fixed at 0·5 by mole. Catalyst concentration was 2 Ã 10É3 mol%. M1 was
w
measured by GPC using chloroform as solvent at 25¡C.
POLYMER INTERNATIONAL VOL. 43, NO. 3, 1997
Characterization and biodegradation of P(AA-L A)
213
Fig. 1. Infrared spectrum of poly(adipic anhydride-co-D,Llactide) (KBr).
Fig. 4. Molecular weight changes of PAA and P(AA-LA)
versus degradation time in 0É1 M pH 7É4 phosphate bu†er at
37¡C. M1
of PAA, P(AA-LA)-82, P(AA-LA)-64 and P(AAn0
LA)-15 were 1É18 ] 104, 1É31 ] 104, 1É36 ] 104 and
1É61 ] 104, respectively.
Fig. 2. 1H NMR spectrum of poly(adipic anhydride-co-D,Llactide) in CDCl . (a) 1É40È1É52 ppm (CH
of LA,
3
3
COCH CH of AA) ; (b) 2É20 ppm (COCH of AA) ; (c)
2
2
2
5É18 ppm (CH of LA).
Fig. 3. 13C NMR spectrum of poly(adipic anhydride-co-D,Llactide) in CDCl . (a) 16É3 ppm (CH of LA) ; (b) 24É2 ppm
3
3
(COCH CH of AA) ; (c) 33É5 ppm (COCH of AA) ; (d)
2
2
2
68É8 ppm (CH of LA) ; (e) 168É8 ppm (CO of LA) ; (f) 173É9 ppm
(CO of AA).
POLYMER INTERNATIONAL VOL. 43, NO. 3, 1997
of CH of LA and wCOCH CH w of AA. In the 13C
3
2
2
NMR spectrum the bands at 16É3, 68É8 and 168É8 ppm
are due to wCH , wCH and wCOw of LA, respec3
tively ; and the bands at 24É2, 33É5 and 173É9 ppm correspond to wCOCH CH w, wCOCH w and wCOw
2
2
2
of AA, respectively.
The DSC data for P(AA-LA) are given in Table 1.
The morphology of P(AA-LA) is related to the compositions of the copolymers. If the AA content in the
copolymers was less than 30 mol%, there was only one
glass transition temperature (amorphous state).
However, the copolymers containing more than
45 mol% AA exhibited melting peaks (semicrystalline
state). The values of T and T for P(AA-LA) were lower
g
m
Fig. 5. Degradation proÐles of poly(adipic anhydride-co-D,Llactide) in 0É1 M pH 7É4 phosphate bu†er at 37¡C.
214
K. J. Zhu, Y . L ei
Fig. 6. Scanning electron micrographs of surface morphology of P(AA-LA)-82, after (a) 0 day, (b) 3 days, (c) 10 days and (d) 16
days in 0É1 M pH 7É4 phosphate bu†er at 37¡C.
than those of the homopolymers. This may be advantageous for the formulation of unstable drugs, such as
proteins, under mild conditions.
In vitro degradation
The copolymers su†ered hydrolysis and dissolution in
saline solution ; this process can be followed by determination of the molecular weight and weight loss of the
matrix. Figures 4 and 5 show the molecular weight
changes and weight loss of PAA and P(AA-LA) in the
test period, respectively. It can be seen that the molecular weight of PAA, P(AA-LA)-82 and P(AA-LA)-64
decreased rapidly during the Ðrst day (more than 70%),
while P(AA-LA)-15 only dropped by 15%, then all
slowed down. On the other hand, the matrix of PAA
was completely degraded in 20 days, but for P(AA-LA)-
15 only 10% mass was lost in the same period. The
weight loss of PAA and P(AA-LA) copolymers showed
a near-constant rate of degradation after a rapid loss in
the Ðrst day, which was caused by dissolution of low
molecular weight fractions. One contributing factor to
this degradation behaviour may be the chemical structure of the copolymer, containing anhydride and ester
groups in the backbone. It is known that anhydride
groups are more susceptible to hydrolysis in water than
ester groups. However, upon hydrolysis of the end
groups, wCOOH groups were formed. This local acidic
environment decreases the anhydride degradation13 but
accelerates ester degradation.14 It may lead to constant
degradation of the copolymers in the later stages.
The external shape of the disc samples remained
unchanged even after 16 days, although the sample disintegrated after removal from the vial unless handled
POLYMER INTERNATIONAL VOL. 43, NO. 3, 1997
Characterization and biodegradation of P(AA-L A)
Fig. 7. pH dependence of weight loss of P(AA-LA)-82 in
saline solution at 37¡C.
215
Fig. 10. Release of BSA and NET (15% loading) from P(AALA)-15 in 0É1 M pH 7É4 phosphate bu†er at 37¡C.
with great care. The surface appearance of the copolymers was observed with a scanning electron microscope.
The surface of P(AA-LA)-82 was smooth and regular at
the initial stage, but after 16 days in saline solution deep
cracks had appeared (Fig. 6).
The pH e†ect on the degradation rate of the copolymers is as shown in Fig. 7. With increasing pH values
the degradation rate was increased, in accordance with
pH dependence of degradation of aromatic polyanhydrides.13
In vitro drug release
Fig. 8. Release of BSA (15% loading) from PAA and P(AALA) copolymers in 0É1 M pH 7É4 phosphate bu†er at 37¡C.
The release proÐles of model drugs are shown in Fig. 8.
The release rate depended on the composition of the
copolymers and basically followed zero-order kinetics
after an initial rapid release. Furthermore, a comparison
of BSA with NET shows a resemblance in the release
rate from P(AA-LA)-82, which is synchronized with
weight loss of the matrix (Fig. 9), even though their difference in molecular weight was greater than a factor of
200. These results provide clear evidence of the bioerodible behaviour of the matrices. However, with
increasing LA content in the copolymers, the correlation between release and degradation was no longer
the case. For example, when the LA content was
85 mol% in the copolymers, the drug release rate and
weight loss of the matrix were very di†erent, as shown
in Fig. 10, indicating that the drug release did not
follow the typical behaviour for a bioerodible matrix.
CONCLUSIONS
Fig. 9. Release of BSA and NET (15% loading) from P(AALA)-82 in 0É1 M pH 7É4 phosphate bu†er at 37¡C.
POLYMER INTERNATIONAL VOL. 43, NO. 3, 1997
Poly(adipic-co-D,L-lactide) can be synthesized by ringopening polymerization from adipic anhydride and D,Llactide using a number of catalysts, such as Sn(oct) ,
2
AlEt ÈH O and Al(o-iPr) . The properties of the
3 2
3
216
copolymers can be tailored by adjusting the composition. Degradation and drug release tests showed that
typical surface erosion behaviour of the matrices and
zero-order kinetics for drug release were achieved with
higher AA contents of the copolymers. The materials
with lower T and T may be advantageous for formug
m
lation of peptide and protein drugs.
ACKNOWLEDGEMENT
We are indebted to the National Natural Science Foundation of China for their support of this work.
K. J. Zhu, Y . L ei
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REFERENCES
1 Heller, J. & Baker, R. W., in Controlled Release of Bioactive
Materials, ed. R. W. Baker. Academic Press, New York, 1980, pp.
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POLYMER INTERNATIONAL VOL. 43, NO. 3, 1997